Pythonize CScript

Makes it possible to do things like:

script = CScript() + OP_HASH160 + scripthash + OP_EQUAL
Add EvalScript() unittests

The tests aren't actually set to fail currently - they just print their
result to stdout.

script_(in)valid.json is from bdd34642dc7e6d5c2d8c7cf6b53e6041b89bf022